This application claims the priority of Japanese Patent Application No. 2000-050783 filed on Feb. 28, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical system used in an oblique incidence interferometer which makes it possible to measure the planarity of a rough surface, in particular, in a noncontact manner; and an apparatus using the same.
2. Description of the Prior Art
Conventionally, various interferometer apparatus for measuring the planarity of surfaces of processed products have been known. Among them, oblique incidence interferometer apparatus have been known as an apparatus which can measure the planarity of test surfaces having large irregularities.
The oblique incidence interferometer apparatus have been used for measuring the planarity of rough surfaces and the like in a noncontact manner, since their measuring sensitivity can be set lower by making a coherent light beam bundle obliquely incident on a test surface. Letting xcex be the wavelength of light used for measurement, and xcex8 be the angle of incidence with respect to the test surface, the amount of irregularities of the test surface, i.e., the measuring sensitivity xcex94 is represented by the following expression:
xcex94h=xcex/(2 cos xcex8) 
Namely, as the incident angle xcex8 becomes greater, so that the degree of oblique incidence increases, the fringe interval becomes longer, so that the measuring sensitivity lowers, whereby it becomes possible to measure surfaces having a low surface accuracy.
FIG. 14 shows a first configurational example of conventional oblique incidence interferometer apparatus, using a planar reference plate as a reference standard. In this oblique incidence interferometer apparatus, a reference plane 116a of a plane parallel plate 116 and a test surface 2a of a sample 2 are disposed so as to oppose each other. Coherent light emitted from a laser light source 111 is turned into parallel light by a collimator lens 114 and obliquely irradiates the reference plane 116a. Interference fringes corresponding to the optical path difference based on the distance between the reference plane 116a and test surface 2a are projected onto a screen 118, so as to be viewed by an observer 119. In this example, as shown in FIG. 14, reference light and measurement light are separated from each other at the reference plane 116a and then are combined together at this plane.
FIG. 15 is a second configurational example of conventional oblique incidence interferometer apparatus, which is an example known as Abramson type using a right isosceles triangle prism as a reference standard. In FIG. 15 and its subsequent conventional examples, members similar to those of the oblique incidence interferometer apparatus shown in FIG. 14 are referred to with numerals whose lower two digits are the same as those of their equivalents in FIG. 14. In this apparatus, coherent parallel light is made incident on a right isosceles triangle prism 216 from an entrance surface 216b. As in the above-mentioned first example, reference light and measurement light are separated from each other at a reference plane 216a and then are combined together at this plane. This apparatus is configured such that interference fringes projected on a screen 218 are captured by a TV camera 219 so as to be viewed.
FIG. 16 shows a third configurational example of conventional oblique incidence interferometer apparatus, which is an example known as Birch type, using diffraction gratings.
In this oblique incidence interferometer apparatus, coherent parallel light is made incident on a diffraction grating 317a, so that its wavefront is divided into two directions. One of thus obtained light beam bundles is made obliquely incident on a test surface 2a, and the resulting reflected light is used as measurement light, whereas the other light beam bundle is used as reference light. The measurement light and reference light are made incident on a diffraction grating 317b, so as to combine their wavefronts together. Interference fringes generated by the optical interference between the measurement light and reference light emitted from the diffraction grating 317b in the same direction are projected onto a hologram screen 318, and are captured by a TV camera 319 so as to be viewed. In FIG. 16, the zero-order diffracted light subjected to wavefront division at the diffraction grating 317a is used as the reference light, whereas the first-order diffracted light is used as the measurement light. The first-order diffracted light of reference light and the zero-order light of measurement light are combined together at the diffraction grating 317b in a later stage, so as to interfere with each other.
FIG. 17 is a fourth configurational example of conventional oblique incidence interferometer apparatus, which is an example employing a Mach-Zehnder type interferometer as an oblique incidence interferometer.
In this oblique incidence interferometer apparatus, coherent parallel light is divided into two directions by a half mirror 417a. One of thus obtained light beam bundles is used as measurement light so as to be made obliquely incident on a test surface 2a by way of a mirror 415a, whereas the other light beam bundle is used as reference light. The measurement light reflected by the test surface 2a and the reference light reflected by the mirror 415b are combined together by a half mirror 417b. Interference fringes generated by optical interference between the measurement light and reference light are projected onto a screen 418, so as to be viewed directly or by use of a TV camera and the like.
Though optical systems for oblique incidence interferometers having various configurations and apparatus using the same have conventionally been proposed as mentioned above, the optical systems and apparatus have their own problems.
Functions required for the optical systems for oblique incidence interferometers and apparatus include easiness in viewing the interference fringes formed. One of causes obstructing the viewing is interference noise.
For example, in the configuration of the first conventional example, interference noise is likely to occur due to the reflected light at a surface (which may be an entrance or exit surface) of the planar plate other than the reference plane. This interference noise can be reduced to a certain extent if the surface is provided with an antireflection coating. Since the angle of incidence is large, however, a coating having a low reflectivity is hard to provide.
The apparatus of the second conventional example can prevent the interference noise from occurring due to such surface reflection. Nevertheless, a problem of interference noise caused by multi-reflected light between the test surface and reference plane remains even in such a configuration. For eliminating this problem, the multi-reflected light between the test surface and reference plane and the light internally reflected by the reference plane should be prevented from interfering with each other and reaching the screen surface. In order to prevent this from happening with a relative arrangement of optical members, if will be effective if the reference plane is set to a size at least twice that of the test surface, for example. The conventional configurational example using the right isosceles triangle prism 216 is problematic in that the prism 216 itself becomes very large, heavy, and expensive in order to enlarge the reference plane 216a as such.
There is also a problem that the luminous flux internally reflected by the reference plane 216a of prism 216 is emitted into the same direction as the interfering light, whereby the noise caused by this internally reflected light is superimposed on the screen 218.
Since the third conventional example utilizes a diffraction grating, there is a fear of unnecessary orders of diffracted light becoming noise light. Therefore, in order to take out only necessary orders of diffracted light, a method of forming an image of interference fringes by use of a relay lens is employed in practice.
Even in a configuration using half mirrors 417a, 417b as in the fourth conventional example, interference noise occurs due to multiple reflections between the front and back surfaces of each half mirror 417a, 417b and is hard to eliminate.
The third conventional example in which light utilization efficiency becomes worse due to the use of diffraction grating, for example, is problematic not only due to the interference noise but also in terms of easiness in viewing.
The first and second conventional examples in which the reference plane 116a, 216a is disposed considerably close to the test surface 2a although not in contact therewith, for example, are more restrictive in terms of installing conditions and viewing, whereby the easiness in viewing is likely to deteriorate. The case where the reference plane 116a, 216a is disposed as such is also problematic in terms of configuration in that attention is required when aligning the test surface 2a. 
Further, the optical system and the apparatus as a whole are desired to be made compact.
For example, though the luminous flux directed from the collimator lens 214 to the test surface 2a in the second conventional example must cover both the incident light and reference light components with respect to the test surface 2a, the incident angle with respect to the prism entrance surface 216a is small in this conventional configurational example using the right isosceles triangle prism 216, whereby a parallel luminous flux having a large luminous flux diameter is required to be made incident. Therefore, the collimator lens 214 also becomes larger, whereby the interferometer as a whole inevitably increases its size.
In the third conventional example, a large quantity of light is lost due to the use of diffraction gratings, whereby a hologram screen or the above-mentioned relay lens is used for favorably viewing the interference fringes, which may increase the size and cost of the apparatus.
In view of such circumstances, it is an object of the present invention to provide an optical system for an oblique incidence interferometer, whose light quantity loss and interference noise are low, which is easy to view interference fringes and carry out alignment, and can make the optical system and the whole apparatus compact; and an apparatus using the same.
The present invention provides an optical system for an oblique incidence interferometer, in which collimated coherent light is divided by luminous flux dividing means, one of thus obtained luminous fluxes is used as reference light, the other luminous flux is used as measurement light made obliquely incident on a test surface, the reference light and the measurement light reflected by the test surface are combined together by luminous flux combining means so as to interfere with each other, and thus generated interference fringes are formed on an interference fringe observing screen;
wherein the luminous flux dividing means comprises a first prism, the reference light and measurement light being separated from each other at a surface where the coherent light enters the first prism or a surface where the coherent light exits from the first prism; and
wherein the luminous flux combining means comprises a second prism, the reference light and measurement light being combined together at a surface where the measurement light enters the second prism or a surface where the measurement light exits from the second prism.
The apparatus in accordance with the present invention comprises the optical system for an oblique incidence interferometer, and a camera.
Here, xe2x80x9ca surface where the coherent light exits from the first prismxe2x80x9d refers to a surface from which the coherent light exits in the case where it has entered the first prism without being divided at the entrance surface of the first prism.